1. Field of the Invention
The present invention is generally related to communication systems. More particularly, the present invention is related to mapping a sequence of data to a Quadrature Amplitude Modulation (QAM) constellation.
2. Related Art
Quadrature amplitude modulation (QAM) is a digital modulation scheme that conveys data by modulating the amplitude and phase of a sinusoidal carrier signal. A constellation diagram depicts the employed set of discrete modulation values in the complex (or Argand) plane, in which typically the x-axis represents real parts and the y-axis represents imaginary parts. The points of the constellation diagram are usually referred to as the modulation symbols that comprise the modulation alphabet. In a QAM scheme, the constellation points are often arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible. The number of points on the constellation diagram for a QAM scheme is usually a power of two, such as two, four, or eight, to provide some examples. Some of the most common number of points on the constellation diagram are sixteen points representing a 16-QAM, sixty four points representing a 64-QAM, one hundred twenty eight points representing a 128-QAM, and two hundred fifty six points representing a 256-QAM. By moving to a higher-order constellation, such as 1024-QAM to provide an example, more bits per symbol can be transmitted. However, for a given average energy per symbol the constellation points in a higher-order constellation must be closer together. This makes discrimination between the modulation symbols more susceptible to noise and other signal corruptions, and thus can result in a higher error rate.
Commonly, the reliability of symbol detection in the receiver can be increased by restricting the permitted sequence of QAM symbols by an error correction code. In this case, an encoder adds redundancies to the sequence of bits, which are then mapped into QAM symbols. A receiver can then exploit the redundancies to improve the correct determination of the transmitted sequence of modulation symbols. Among various error-correction codes, low density parity check (LDPC) codes are particularly well suited for achieving performance very close to the capacity of a given communication channel. However, very long LDPC codes requiring complex encoders and decoders are needed to obtain good results for higher-order QAM, if all constellation bits are encoded.
Thus, there is a need for an apparatus and/or a method to encode data into higher-order QAM symbols by a LDPC code that overcomes the shortcomings described above. Further aspects and advantages of the present invention will become apparent from the detailed description that follows.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.